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STELLAR EVOLUTION til 2002 none of the researchers who had been studying radiative levitation in white dwarfs attached the right importance to a possible external origin of the metals. The only cases in which this possibility was taken into consideration were related to particularly cold, and/or metal-rich white dwarfs for which the radiative levitation mechanism was not sufficient to account for what’s observed. As external source of those metals it was simply cho- sen the interstellar medium, or ra- ther, its local concentrations of gas and dust that exist in the space be- tween the stars, which would oc- casionally transfer into the atmo- spheres in question. This solution though set off at least two objec- tions: 1) the white dwarfs with ex- cesses of metals (compared to theoretical predictions) are not located in close proxim- ity to condensations of the interstellar me- dium, and if in the past they ever crossed any, the metals acquired would have been long ago dispersed in the subsurface layers R endering of the Far Ultra- violet Spectro- scopic Explorer, the instrument which gathered the data used by the team led by Martin Barstow to identify the source of the met- als present in the atmospheres of white dwarfs. [JHU FUSE Project] Left: Professor Martin Barstow of the University of Leicester. (especially in cooler white dwarfs); 2) the white dwarfs with atmospheres dominated by helium show a very unbalanced ratio be- tween hydrogen and calcium in favour of the latter, while it should instead reflect the proportion between those elements in the interstellar medium where hydrogen is de- cidedly the most abundant. The pieces of the puzzle started to come together in recent years with the discovery of metal-rich white dwarfs, surrounded at the same time by or- biting rings of dust and gas (visible in the in- frared). It is precisely this material compos- ing the rings that infalling into the atmo- spheres of white dwarfs brings the metals detected. But what is its origin? According to a recent work by Martin Barstow and others, published in the Monthly Notices of the Royal Astronomical Society , to form that material would have been the remains of rocky planets orbiting around stars before their transformation into white dwarfs. The stellar disruptions leading to that final evo- lutionary state can cause planetary orbits to decay and, as a result, bring the planets at short distances from white dwarfs. If those distances are less than the Roche limit (about 1 solar radius in this case), planets are torn apart and their debris (ranging in size be- tween those of dust particles and those of small asteroids) will eventually line up along